Advanced D3D10 Rendering
Emil Persson
May 24, 2007
Advanced D3D10 Rendering2 May 24, 2007
Overview
Introduction to D3D10
Rendering techniques in D3D10
Optimizations
Advanced D3D10 Rendering3 May 24, 2007
Introduction
Best D3D revision yet! Clean and powerful API Lots of new features
– SM 4.0 New geometry shader Stream Out
– Texture arrays
– Render to volume texture
– MSAA individual sample access
– Constant buffers
– Sampler state decoupled from texture unit
– Dual-source blending
– Etc…
Advanced D3D10 Rendering4 May 24, 2007
Clean API
Vista only Everything is mandatory (almost)
– No legacy hardware support
Clean starting point for future evolution of the API– Limited market short-term
Some old features deprecated– Fixed function
– Assembly shaders
– Alpha test
– Triangle fans
– Point sprites
– Clip planes
Advanced D3D10 Rendering5 May 24, 2007
Dealing with deprecated features
Fixed function– Write a few über-shaders
Assembly shaders– Convert to HLSL
Alpha test– Use discard or clip() in pixel shader
– Use alpha-to-coverage
Triangle fans– Seldom used anyway, usually just for a quad
– Convert to triangle list or strip
Point sprites– Expand point to 2 triangles in GS
Clip planes– Use clip distance and/or cull distance
Advanced D3D10 Rendering6 May 24, 2007
SM 4.0
Geometry shader
– Processes a full primitive (point, line, triangle)
– Has access to adjacency information (optional) Useful for silhouette detection, shadow volume extrusion etc.
– May output multiple primitives Output limitation is 1024 floats
– May output nothing (to kill primitive)
Advanced D3D10 Rendering7 May 24, 2007
SM 4.0
Infinite instruction count– Very long shaders may have lower throughput though
Integer and bitwise instruction Indexable temporaries
– Allows for local arrays– May be used to emulate a stack
Useful system generated values– SV_VertexID– SV_PrimitiveID– SV_InstanceID– SV_Position (Like VPOS, but now .zw are defined too)– SV_IsFrontFace (Like VFACE)– SV_RenderTargetArrayIndex– SV_ViewportArrayIndex– SV_ClipDistance– SV_CullDistance
Advanced D3D10 Rendering8 May 24, 2007
SM 4.0
Integer & bitwise instructions
– Signed and unsigned No idiv though, just udiv
– Same registers as floats Can alias without conversion with asint(), asuint(), asfloat() etc.
– Integer texture sample values Syntax: Texture2D <uint4> myTex;
Access to individual samples in MSAA surface
– Allows for custom AA resolve Syntax: Texture2DMS <float4, 4> myTex;
Advanced D3D10 Rendering9 May 24, 2007
Pixel center
Half pixel offset is gone! – Affects SV_Position as well
– Now matches OpenGL
DX10 DX9
Advanced D3D10 Rendering10 May 24, 2007
Pixel center
Pixels and texels align
– TexCoord = SV_Position.xy / float2(width, height)
Texel center Screenspace
Advanced D3D10 Rendering11 May 24, 2007
The small batch problem
D3D10 designed to minimize batch overhead
– Pulls work from draw time to creation time Validation
Shader input/output configuration
– Immutable State Objects Input layout
Rasterizer state
Sampler state
Depth stencil state
Blend state
Advanced D3D10 Rendering12 May 24, 2007
The small batch problem
D3D10 also provides tools to reduce draw calls
– Improved instancing interface
– Geometry shader
– More shader resources
– Constant indexing in PS
– Render target arrays
– Texture arrays
Advanced D3D10 Rendering13 May 24, 2007
Rendering techniques in D3D10
Advanced D3D10 Rendering14 May 24, 2007
Global Illumination
Advanced D3D10 Rendering15 May 24, 2007
Global Illumination
Probes on a volume grid across the scene
– Each probe captures light environment into a tiny “cubemap”
– Probes are converted to Spherical Harmonics coefficients
Indirect lighting is computed using interpolated SH coefficients
– Do the same in probe passes to get multiple light bounces
Advanced D3D10 Rendering16 May 24, 2007
Global Illumination
Awful lot of work
– Each probe is 6 slices. We need loads of probes.
– Sample scene has over 300 probes
Solution
– Use geometry shader to reduce work
– Distribute work across multiple frames Sample updates 40 cubes per frame
Scatter updates to hide artifacts
– Skip over “empty” space probes
Advanced D3D10 Rendering17 May 24, 2007
Global Illumination
The Geometry Shader advantage
– 40 cubes x 6 faces x n draw calls = Pain DX9 style unrealistic even for simple scenes
– Update multiple slices per pass with GS GS output limit is 1024 floats
Keep number of interpolators down to maximize primitive count
Managed to update 5 probes (30 slices) per pass
8 passes is more manageable than 240 ...
Advanced D3D10 Rendering18 May 24, 2007
Post tone-mapping resolve
D3D10 allows for custom AA resolves
– Can drastically improve HDR AA quality Standard resolve occurs before tone-mapping
Ideally resolve should be done after tone-mapping
Standard resolve Custom resolve
Advanced D3D10 Rendering19 May 24, 2007
Post-tonemapping resolve
Texture2DMS<float4, SAMPLES> tHDR;
float4 main(float4 pos: SV_Position) : SV_Target { int3 coord; coord.xy = (int2) pos.xy; coord.z = 0;
// Tone-map individual samples and sum it up float4 sum = 0; [unroll] for (int i = 0; i < SAMPLES; i++) { float4 c = tHDR.Load(coord, i); sum.rgb += 1.0 – exp2(-exposure * c.rgb); } // Average sum *= (1.0 / SAMPLES);
// sRGB sum.rgb = pow(sum.rgb, 1.0 / 2.2);
return sum; }
Advanced D3D10 Rendering20 May 24, 2007
Optimizations
Advanced D3D10 Rendering21 May 24, 2007
Geometry shader
GS optimizations
– Input/output usually the bottleneck
– Reduce outputs with frustum and/or backface culling
– Keep input small by packing data TexCoord could be 2x16 bits in an uint
– Or use for instance asuint(normal.w)
Merge to full float4 vectors
– Don’t do 2x float2
– Keep output small Could be faster to trade for some work in PS
Pass just position, don’t interpolate both lightVec and viewVec
– Or even back-project SV_Position.xyz to world space in PS
Small output means more work fits within 1024 floats limit
Advanced D3D10 Rendering22 May 24, 2007
GS frustum and backface culling
// Transform to clip space
float4 pos[3];
pos[0] = mul(mvp, In[0].pos);
pos[1] = mul(mvp, In[1].pos);
pos[2] = mul(mvp, In[2].pos);
// Use frustum culling to improve performance
float4 t0 = saturate(pos[0].xyxy * float4(-1, -1, 1, 1) - pos[0].w);
float4 t1 = saturate(pos[1].xyxy * float4(-1, -1, 1, 1) - pos[1].w);
float4 t2 = saturate(pos[2].xyxy * float4(-1, -1, 1, 1) - pos[2].w);
float4 t = t0 * t1 * t2;
[branch]
if (!any(t))
{
// Use backface culling to improve performance
float2 d0 = pos[1].xy * pos[0].w - pos[0].xy * pos[1].w;
float2 d1 = pos[2].xy * pos[0].w - pos[0].xy * pos[2].w;
[branch]
if (d1.x * d0.y > d0.x * d1.y || min(min(pos[0].w, pos[1].w), pos[2].w) < 0.0)
{
// Output primitive here ...
}
}
Advanced D3D10 Rendering23 May 24, 2007
Miscellaneous optimizations
Pre-baked constant buffers
– Don’t update per-material constants in DX9 style
PS don’t need to return float4 anymore
– Use float3 if you only care about RGB
– May reduce instruction count
Use GS to reduce draw calls
– Single pass render-to-cubemap
– Update multiple render targets per pass
Advanced D3D10 Rendering24 May 24, 2007
The new shader compiler
SM4 shader compiler preserves semantics better
– This means more responsibility for you guys
– Be careful about your assumptions
– Periodically check the resulting assembly D3D10DisassembleShader()
Use GPUShaderAnalyzer for performance critical shaders
Advanced D3D10 Rendering25 May 24, 2007
The new shader compiler
HLSL code: float4 main(float4 t: TEXCOORD0) : SV_Target { if (t.x > t.y) return t.xyzw; else return t.wzyx; }
DX9 assembly: add r0.x, -v0.x, v0.y cmp oC0, r0.x, v0.wzyx, v0
DX10 assembly: lt r0.x, v0.y, v0.x if_nz r0.x // <--- Did you really want a branch here? mov o0.xyzw, v0.xyzw ret else mov o0.xyzw, v0.wzyx ret endif
Example:
Advanced D3D10 Rendering26 May 24, 2007
The new shader compiler
Use [branch], [flatten], [unroll] & [loop] to control output code– This is not for everyone– Poor use could reduce performance
Make sure you know what you’re doing Only use if you’re familiar with assembly code Verify that you get the code you expect
– Always benchmark both options
New DX10 assembly (using [flatten]): lt r0.x, v0.y, v0.x movc o0.xyzw, r0.xxxx, v0.xyzw, v0.wzyx ret